A typical gas chromatograph 10 is illustrated in FIG. 1 and includes a column 18 positioned within an oven 24. Chromatographic separation of a sample 20 is accomplished by injecting the sample 20 into a pressurized carrier gas 13 through an injection port 12 into the column 18. Either manual control of pressure valve 14 or electronic pneumatic control provides for adjusting the pressure of the carrier gas 13 at the head of the column 18 (hereinafter "column head pressure") in response to a valve control signal 15. A pressure transducer 16 generates a pressure information signal 17 proportional to the column head pressure; this signal 17 is transmitted to controller 42 (FIG. 2). A heater 26 provides heat to the oven 24 in response to internal control signals (not shown) or to a heater control signal 27 from computer 40. A temperature sensor 28 generates a temperature signal 29 proportional to the temperature in the oven 24; the signal 29 is transmitted to computer 40. The carrier gas/sample combination passing through column 18 is exposed to a temperature profile resulting from the operation of the heater 26 within oven 24. There is a direct correlation between the temperature profile and the retention time of different compounds making up the sample in the column. As compounds elute from the column 18, a detector 30 generates an electrical signal corresponding to some characteristic of the compounds. The non-analyzed portion of the carrier gas/sample combination passes through a mass flow controller 22 to vent 21. FIG. 2 illustrates the electronics employed for controlling the GC, including keypad 38, computer 40, and controller 42. Computer 40 includes a central processing unit, memory 41, and associated devices, such as random access memories, read-only memories, input/output isolation devices, and other components. One of the functions of computer 40 is to control the temperature of oven 24 by monitoring temperature signal 29 from sensor 28 and transmitting the appropriate heater control signal 27 to heater 26. For electronic pneumatic control, one of the functions of controller 42 is to control the column head pressure by monitoring pressure signal 17 from transducer 16 and transmitting the appropriate valve control signal 15 to valve 14.
Chromatographers spend a significant portion of their time developing and optimizing chromatographic methods for the separation, identification, and quantification of specific compounds within a sample. Such a chromatographic method will specify the use of a column having defined column parameters (e.g., length, stationary phase, and inside diameter) as well as operating parameters for the gas chromatograph (e.g., carrier gas type and pressure, oven temperature, and ramp rates). A compilation of chromatographic methods entitled "Analytical Solutions--a Collection of Chromatograms from Hewlett-Packard" is commercially available from the Hewlett-Packard Company, Palo Alto, Calif. This compilation contains a collection of over 400 capillary column methods developed by Hewlett-Packard field and factory personnel over a period of ten years. FIG. 3 illustrates one such method entitled "Drug Standard (2)" which is employed for identifying drugs and highlights the column parameters and the operating parameters required to replicate this chromatographic method. The column parameters include: column length (l), column inside diameter (id), stationary phase thickness (d.sub.f), and stationary phase type). The operating parameters include: carrier gas type, pressure and/or flow rate, and oven temperature (including program ramp rates). A chromatogram highlighting the retention times at which known compounds will elute from the column 18 makes it possible to identify unknown compounds based on the elution order and time.
The chromatographer reviews compilations of chromatographic methods to obtain one having the greatest potential for determining unknown amounts and identities of compounds in samples. However, it is generally not possible to change only one column or operating parameter without affecting the retention time of each compound, resolution, and in many cases, the elution order of each compound. The vast number of column types and nominal sizes makes it quite possible that a chromatographer will be unable to practice a published chromatographic method for lack of a column that matches the column specified in the chromatographic method.
U.S. Pat. No. 5,405,432 entitled "Capillary Column Method Translation" to Snyder et al, and hereby incorporated by reference, teaches a method for translating a known "original" chromatographic method specifying a set of column and operational parameters into a new chromatographic method having possibly new column and/or new operational parameters and a chromatographic output with similar separation. System software running on the gas chromatograph or a remote computer controls a gas chromatograph to automatically reconfigure with the new operational parameters. The existing column and operating parameters are stored such that upon input of the new column parameters, system software has all of the information necessary to perform translation calculations required to obtain the new operating parameters. Once the new operating parameters are calculated, the gas chromatograph is reconfigured such that the chromatographic output of the gas chromatograph with the new column and new operating parameters is substantially the same as with the old column and old operating parameters. However, while elution order may be maintained with this approach, retention times are not locked to those of the original chromatographic method.
A chromatographic method is typically developed with the intent that a chromatographer will be able to practice the method. A problem facing all chromatographers is the inability to exactly replicate column and operating parameters in accordance with a prescribed or translated chromatographic method. Without exact replication, measured retention times do not match the retention times specified in the original chromatographic method or the computerized method files (including calibration and event tables) and can lead to misidentified peaks with grave consequences in drug analysis, enviromental, or petrochemical applications.
Sophisticated integration software (for example, the Hewlett-Packard 3365 ChemStation software available from Hewlett-Packard Company, Palo Alto, Calif.) may be employed, in combination with a preliminary sample injection of a "standard" having known retention times, to update the corresponding calibration table by identifying one or several large and isolated chromatographic peaks as references. However, in order to identify these reference peaks, they must fall within a retention time "reference window". If each reference peak occurs during the window, then the integration software automatically recalibrates to the new retention times by a proportional amount. However, if the retention times of the reference peaks occur outside the reference windows, or if the elution order of any of the standard peaks changes, then the operator must identify patterns of reference peaks and override incorrect peak identifications. While it is possible to broaden or move the reference windows, it can be time consuming and is difficult to automate, especially for the analysis of complex samples having many analytes. Additionally, sufficient broadening of reference windows may cause overlapping windows and misidentification of peaks.
Even assuming that a gas chromatograph has been set up in accordance with a chromatographic method and the retention times of the calibration sample have been calibrated, there exists an ongoing need for recalibration. Deposits of non-volatile compounds at the head of the column which tend to contaminate the column, shift retention times and distort peak shapes necessitating the removal of a portion of the column. The column then exhibits retention times that, under the original operating parameters, are shorter.
There is a need to quickly and easily replicate a method such that it may be used by others without time-consuming calibration and re-calibration upon the modification or replacement of a column without the additional large number of calibration steps required with retention indices. Additionally, it would be advantageous not to require the correction/adjustment of runtime tables (integration, timed events etc.)
It would be advantageous to adjust the operating parameters of a gas chromatograph after a new column is installed in accordance with a new chromatographic method such that the retention times of all compounds match those defined in the original chromatographic method, without using tedious indices calibrations. In particular, this would simplify installation and validation of the new method chromatographic; reduce the cost of setting up methods; reduce errors associated with changes in the calibration; and provide for automation of the set up procedure in combination with electronic pneumatics control, or manual with mechanical control if necessary.
There is a need for a method for predictably translating a known chromatographic method to work with a column having different column parameters such that retention times obtained by the original column are predictably recast with no change in the order of elution.
Another object of the invention is to incorporate retention time locking into a gas chromatograph that automatically translates known methods to accommodate columns of diameters or lengths, as well as operational parameters that vary from a known chromatographic method, and yet maintain retention time locking. In particular, it is desirable to have the gas chromatograph automatically configure with new operating parameters such that the chromatographic output of the gas chromatograph with the new column and operating parameters is the same as with the original column and operating parameters.
It would be desirable and of considerable advantage to lock the retention times of the new column to match the retention times of the original column without numerous recalibrations, so as to provide an improvement in one or more of the following: substitution of any available column having the same stationary phase coating, the ability to provide for reoptimization and direct comparability to existing chromatographic methods, provide for a predictable improvement in analysis speed, capacity, improved troubleshooting and diagnostics, increased remote support ability, accuracy of quantification or resolution, and the identification of compounds.